it's good to install UsbBoot-Installer on the same system you intend to usb3 boot your win7, although not a must
it's recommended to rename C: \WINDOWS\System32\drivers\pcw.sys as it may cause issues with non-regular, i.e. usb3 boots. it's just a check system daemon, not seriously needed.
after installation, ahci (normal) reboot and always inspect the C: \WINDOWS\System32\UsbBootWatcher.conf (make sure all your usb3 drivers are properly listed there)
it's recommended to remove/clean your system from any old/unused programs/drivers
I use Autoruns to trim out & clean any non-essential boot devices
always check your usb3/sata cable. some may be poor quality and cause issues.
always check the quality of your usb3 port. some ports tend to easily break and cause issues